I would call them ledges. They have loose material on them that sits at the angle of repose (more or less). They represent more easily erodable material.

I would call them ledges, too. Bdforster's Message 67 mentioned terraces and until I looked terraces up I had the definition wrong in my mind.

The roughly 45 degree angle of these terraces would be caused by till ...

Well, talus...

I think I caught that error, too. Whew, it's probably been a year since I last participated in a geology thread at any level of detail, the terminology fades quickly!

I understand that differential erosion rates cause different angles of slope retreat, but vertical slopes indicate rapid erosion, possibly even a series of waterfalls moving gradually upstream by a few inches per year.

Actually, the other way around. Quickly eroding shales form things like Badlands or the Tonto platform. Resistant rocks form cliffs like the Palisades.

But in the context of canyon slopes, the angle of the slope is a function of downcutting rate versus rate of slope retreat. For any given type of layer, downcutting rate will be a function of rate and nature of water flow, while slope retreat will be relatively constant. A vertical slope can only mean that downcutting was relatively rapid as compared to slope retreat.

Is the type of layer, basalt versus sedimentary, apparent to you from the photo?

You will notice that I carefully called them 'interflow deposits' in my original post.

Oh, yes, I very much did notice. Notice that my reply isn't to you, but to Bdfoster replying to me replying to you. If you look at my reply to you (Message 66) you'll see I wasn't quite certain what you meant by interflow deposits, but correctly guessed that sedimentary deposits was one of the possibilities.

They may be sediments, they may be breccias,...

Breccias ("sediment consisting of angular fragments in a matrix of finer particles," for those of you not sitting with a geology glossary in front of you) seem unlikely, don't they?

...they may be simply porous flow-tops. The added feature they would have is that water would reach daylight along these porous horizons so that plants could live there.

Flow-tops? Not in my glossary, not at Wikipedia, okay, I give up. Water bubbling up from underground?

What I'm trying to figure out is the sequence of events that gave rise to successive ledges. Here's the picture again:

Focusing on the top cliff on the left hand side, the river eroded straight down through the basalt layers, that much seems clear. Okay so far?

Below the cliff is a ledge. What happened to cause the ledge if not a break in uplift during which slope retreat continued? (I know in your next message you say it wasn't uplift, I'll address that in my next message.)

Then how did the river come to be a couple hundred feet below the level of the land? Subsidence of an adjacent region into which the river flows with backcutting upriver of a series of waterfalls and/or fast-flowing sections?

What actually happens is the same thing as the Grand Canyon. Weaker layers erode, thereby undermining the stronger layers. The stronger layers, with their vertical joint systems simply collaps along those joints. So, when erosion happens it is quite fast. The problem is that it happens episodically and, in the long run, is slow. This is part of the confusion regarding the Grand Canyon as well. That is why the erosion looks catastrophic, but still takes a long time to form the overall landform.

This still gives me no picture of how the ledges formed. Probably the information is there, I just don't know how to thread it all together into a cohesive story.

Say you commissioned me to do an animation video showing how the river formed the ledges. What would you tell me to put in the animation?

quote:Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, ash or cinder; breccia and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidised iron from weathered iron-rich minerals such as pyroxene.

As for the vertical sides of the basalt sections:

quote:During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns.

There are several pictures of this in the article, and you can see vertical striations in your picture, evidence of this kind of formation. This would mean that the basalt sections would tend to fracture off in vertical bits as the support ledges immediately below them eroded from underneath.

What I'm trying to figure out is the sequence of events that gave rise to successive ledges. Here's the picture again:

So as the river cuts deeper the soft soils\breccia sections between the basalt layers are exposed to erosion and will erode out, the basalt column formations over those eroded soils\breccia sections will fail and you get the stepped formation of ledges and cliffs getting wider over time.

This does not explain the single straight section, and I would like to know if there is a fold or fault that this section of the river follows. Looking at the GOOGLE MAPS (Palouse Falls, WA) you can see the straight section (close the flyout menu) as well as several other straight line features in the area.

The portion from Wikipedia doesn't help much, because it doesn't explain why Edge would say, "...they may be simply porous flow-tops. The added feature they would have is that water would reach daylight along these porous horizons so that plants could live there." Why would water bubble up a hundred feet above river level, and even if it did, what is the significance for this scenario?

RAZD writes:

So as the river cuts deeper the soft soils\breccia sections between the basalt layers are exposed to erosion and will erode out, the basalt column formations over those eroded soils\breccia sections will fail and you get the stepped formation of ledges and cliffs getting wider over time.

Except that this:

Doesn't resemble the upper left cliff face, the one I've been focusing on, at all:

Then how did the river come to be a couple hundred feet below the level of the land? Subsidence of an adjacent region into which the river flows with backcutting upriver of a series of waterfalls and/or fast-flowing sections?

This region was not significant uplifted but filled in.

The region experienced massive infilling of cubic miles of extremely fluid lava flows that filled in former valleys and drainages so that only the highest summits and ridges remained. In some areas the basalt is miles thick. In fact, little is known of the basement formations below the basalt in vast regions because the coverage is so complete and deep.

The Columbia river with the help of glacial outflows and dozens of Missoula super floods cut new channels thru the young basalt on its way to the Pacific.

Percy writes:

This still gives me no picture of how the ledges formed.

The ledges probably formed because the various layers are distinct with a contact zone that often consists of ash (probably from a preruption activity), sediments, palagonite (result of the meeting of lava and water). As the river or floods cut through basalt they would erode differentially between the layers due to this discontinuity and form ledges? Just a guess.

Doesn't resemble the upper left cliff face, the one I've been focusing on, at all:

There do not appear to be any vertical faults, only horizontal stratification between layers.

Sometimes the columnar jointing is not well developed in lava flows and the one pictured is obviously a textbook example. Few things in geology are straightforward becasue of the enormous number of variables present. I agree there is some horizontal banding in the flows pictured, along with the obvious stratification, but if you look at the right side of the canyon the upper flows do have a columnar appearance. I can virtually guarantee that there is some kind of vertical cooling joint in these rocks.

The ledges probably formed because the various layers are distinct with a contact zone that often consists of ash (probably from a preruption activity), sediments, palagonite (result of the meeting of lava and water).

As an example, just look at the tops of flows in Hawaii. They are a jumbled terrane of blocks, bombs, ash, soils and even vegetation. This is what the 'interflow' material is commonly made up of. It is very uncommon to get a sequence of volcanic rocks that does not have strong vertical variability. That is why the rate of downcutting relative to widening of the canyon is oversimplified for this example: there are multiple rock types.

As the river or floods cut through basalt they would erode differentially between the layers due to this discontinuity and form ledges? Just a guess.

Correct. Rapid erosion of the weaker layers will undermine the stronger one, which in this case, just happen to have vertical planes of weakness (cooling fractures).

Okay, so you and Iceage agree the downcutting wasn't caused by uplift. This is Anglagard's point 21 from Message 46:

River meanders â€“ River meanders incised in rock can only be caused by gradual uplift, not through a yearâ€™s worth of soft sediment deposition.

But here we have a clear example of a river meander incised into rock that was not caused by gradual uplift, so Anglagard's point 21 is wrong. And apologizing in advance, I still don't understand the explanation for what caused this meandering downcutting. Iceage said in Message 80:

Iceage in Message 80 writes:

The Columbia river with the help of glacial outflows and dozens of Missoula super floods cut new channels thru the young basalt on its way to the Pacific.

I don't believe it is. The Palouse River canyon is not an incised meander river canyon.

The river channel below the falls, where it is a straight shot for miles is a good example what to expect from a catastrophic flood (in this case the Missoula floods). The Missoula flood diverted the older Palouse river course forming a 90 degree diversion and made a relatively straight shot to the Snake .

The Goosenecks are entrenched, and has maintained its basic shape for a very long time and with a decidedly V shape, while the Palouse has significantly milder radiused bends and is more U shaped with a local floodplain. All rivers have bends of some degree as due to river hydrological factors.

I don't believe it is. The Palouse River canyon is not an incised meander river canyon.

Okay, looking at the satellite photos I see that Palouse River has no tight meanders, and in fact the spectacular pictures of the canyon are just a small section of the river. It wanders a bit like any river, but there's nothing even close to gooseneck meandering. And the satellite photos make it appear that through much of its course it has produced a flood plain into which it has downcut, though the quality of the photos make it difficult to tell.

So this photo you produced earlier of Goosenecks State Park is of an incised meandering river:

The portion from Wikipedia doesn't help much, because it doesn't explain why Edge would say, "...they may be simply porous flow-tops. The added feature they would have is that water would reach daylight along these porous horizons ...

Maybe I'm wrong, but I read the wiki article to say that the basalt in the tops (ie flow-tops) would be porous and mixed:

quote:Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock.

vesicle - n 3. A small cavity formed in volcanic rock by entrapment of a gas bubble during solidification.

If these sections are porous (frothy bubbles) compared to the lower basalt layers then aquifers could flow along them, but I would think the vertical fissures of the basalt would work against this. It could be that the groundwater is flowing down inside AND horizontally along the interface between the different layers.

Except that this:Doesn't resemble the upper left cliff face, the one I've been focusing on, at all:

I look at the upper left and see clear vertical lines, I look at the second level left and see vertical lines. I agree that they are less distinct than the wiki example, but part of this is due to scale: the wiki shot is up close and personal while the Palouse River is panoramic and from a much greater distance.

I don't know if you can blow sections of the picture up in your browser to see the ledge cliffs in closer detail, but when I use 200% in foxfire print preview I see the top two cliffs on the left showing very definite vertical fissures and columns, and similar elements in the third down layer.

Maybe I'm wrong, but I read the wiki article to say that the basalt in the tops (ie flow-tops) would be porous and mixed:

Well, remembering that a flow top here could also be a flow bottom...

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock.

vesicle - n 3. A small cavity formed in volcanic rock by entrapment of a gas bubble during solidification.

If these sections are porous (frothy bubbles) compared to the lower basalt layers then aquifers could flow along them, but I would think the vertical fissures of the basalt would work against this.

From personal experience this is not always the case. On the Modoc Plateau a few years ago we considered the vertical permeability to be high, but direct observation of lakes showed that this was not so. I imagine that the cooling fractures (columnar joints) are quite tight until they are weathered, or else they plug up easily with silt or oxidation products.

It could be that the groundwater is flowing down inside AND horizontally along the interface between the different layers.

This is almost certainly true as observed in outcrops and in mines. In intact sections, lateral permeabilities are almost always higher than vertical. There are exceptions and they can be discussed if you wish.

This has been really intresting debate guys! I've learned a lot so far. Anyways, if you would be so kind is to read both of these links I provided. ( Considering I've read everyone's links they provided me, I'd be happy if you do the same for me ) I see evidence for a worldwide flood in these links. Also, the last link is a 100 Evidence for a Global Flood. Read the first link before you read the last one. If you see any problems in these links, quote em! I'd be happy to see what's wrong with them. Most likely you'll find somthing ;)

Please read the rules again, Rep - #4 and #5 specifically. Don't argue by link: instead, pick some of the arguments in those links that you think are particularly strong. Post them, with the link, of course, and then explain in your own words why you think the point is persuasive.